The present invention relates to a motor control device and power control device.
Generally, power conversion devices are driven by Pulse Width Modulation (hereinafter referred to as "PWM") signals obtained by comparing a voltage command value and a fixed frequency triangular-shaped carrier wave and are configured to generate a prescribed voltage.
Further, motors can be made small when they are driven by a power conversion device having a high frequency output current. However, in this case, beat phenomena oscillating at a low frequency can occur in the output current of power conversion device, because the frequency of output current is in the region of 1/10 to 1/3 of the carrier frequency of the power conversion device.
In a known method of resolving this problem, the frequency of the carrier is set at a multiple of the output frequency of the power conversion device, (particularly, an odd number multiple, such as 1, 3, 5, 9), and the motor is driven with the PWM signal in synchronism with the output frequency. (This is referred to in the following as the "synchronous PWM method").
This is disclosed in, for example, Japanese Patent Laid-open Publication No's. Hei. 7(1995)-227085, Hei. 7(1995)-67350 and Hei. 6(1994)-197550. These are a first group of example publications where V/F fixed control is carried out so that a ratio of motor voltage and frequency is approximately constant.
Moreover, in "HIGH PERFORMANCE VECTOR CONTROLLED THREE-LEVEL GTO INVERTER SYSTEM FOR ELECTRIC TRACTION" listed in Proceedings of 1995 International Power Electronics Conference (IPEC-Yokohama '95) a method is shown where torque of an "alternating current" (hereinafter abbreviated to "a.c.") motor is controlled using current control while carrying out a synchronous PWM method using space vectors. This method is capable of current control up to higher frequencies when compared with the first group of example publications. This is referred to as a second publication group.
With the synchronous PWM method where carrier waves are compared, when the carrier wave frequency is an even multiple of the output frequency, a distorted wave shape of even harmonic waves becomes superimposed with the output voltage. A third publication example, Japanese Patent Laid-open Publication No. Hei. 6(1994)-197547, discloses well known technology for resolving this problem in synchronous PWM methods employing carrier wave comparisons. Here, the occurrence of even harmonic voltages is suppressed while making the carrier frequency an even-number multiple of the output frequency, by performing inversion or non-inversion control on the PWM signal based on the voltage phase. A finer pulse number can therefore be achieved in the synchronous PWM method, and improvements in the output current waveform can therefore be made.
However, these example publications did not take into account the following points.
The first example publication group, utilizes open loop control of current and speed based on V/F fixed control, and is therefore not suitable for applications in products requiring fine control of motor torque and speed at high speeds. Also, while the carrier frequency for generating the synchronous PWM signal is clearly decided using a speed command value, this becomes complex when carrying out feedback control and can therefore not be applied as is in this case.
The second example publication, uses a PWM signal generating method employing space vectors, and complicated operations for PWM signal generation therefore have to be carried out in an extremely short time. It is therefore necessary to employ a high-performance microprocessor or a digital signal processor capable of high speed arithmetic processing at the control device, which makes the control device expensive.
In the third example publication, a PWM signal-generating processor has to be added to take into consideration dead time for arm short-circuit prevention in order to obtain a PWM signal for driving switching elements of an upper arm and a lower arm of the electric conversion device, which makes the circuit more complex.